split-second
/ˌsplɪtˈsɛk(ə)nd/noun
University
1
A stopwatch with two second hands, each of which may be stopped separately to time the intervals of a race by a sportsperson, or to time more than one sportsperson.
2
A fraction of a second; hence (figurative, also attributive) a very short time period; an instant, a moment.